1. Field of the Invention
The present invention relates to a control unit for an air bag and, in particular, to the control unit for the air bag which increases reliability of an operation of the air bag.
2. Description of the Related Art
In conventional air bag systems, an inflater and a control unit are housed in separated cases, and are connected with each other through a connector or the like. Such a conventional control unit after an inflation of the air bag is not different in appearance from the same before the inflation if it is separated from the inflater. This may lead to reuse of the control unit that has ever operated the inflater.
In the control unit, large electric current may flow when the inflater is ignited. The switch devices or the like therein may be damaged with the large electric currents. This means that the used control unit (i. e. the control unit which has ever made inflater operate) has only little reliability.
A memory unit for the air bag system is disclosed in Japanese Patent Laid-Open No. 18336 in 1997. The memory unit comprises a non-volatile memory device, and a microcomputer which can write acceleration data at certain intervals for the latest certain period into the non-volatile memory device when the air bag is inflated and which can then put the memory device under a ban on writing. According to the art, it is possible to know from the acceleration data written in the memory unit whether the inflation of the air bag was performed normally or abnormally. Additionally, it is possible to also know the delay time between the detection of the certain acceleration and the inflation of the air bag, if any.
However, the control unit for the air bag with the above memory unit after the inflation of the air bag is not different in appearance from the same before the inflation if it is separated from the inflater. This may lead to reuse of the used control unit that has ever operated the inflater.
As measures to prevent reuse of the control units after the inflation, there are operation manuals distributed by manufacturers, which tell operators not to reuse the control units.
However, only operators removing the control unit from the car can know whether it has made inflater operate or not. Therefore, it is possible for the control unit after the inflation to be on the market.